X-ray micro- and nano-computed tomography is a well-known non-destructive technique for visualizing and quantifying the internal structure of objects in three dimensions (3D). It is used to provide high resolution images of rocks in 2D or 3D at a micron scale (see, for example, M. A. Knackstedt et al., “Digital Core Laboratory: Properties of Reservoir Core Derived From 3D Images,” SPE 87009, 2004).
X-ray micro- and nano-computed tomography techniques enable acquisition of grayscale 3D images in which the grayscale represents the X-ray absorption distribution within the object. The absorption (attenuation) depends on the chemical composition of the material and its physical density. The range of X-ray energies used in computed tomography (CT) allows the study of very dense objects, such as rocks.
Strong contrast in X-ray attenuation coefficients between rock grains and air/water/oil-filled pores, the allows the microCT technique reproducing 3D images with dark and bright areas inside. The dark and bright areas correspond to pores and grains respectively (if, as usual, the 3D microCT image is considered at inverse gray scale). The problem is that the boundary between dark and bright regions is not step-like. In order to segment a rather smoothed grayscale microCT image in two phases (grains and pores) it is necessary to apply binarization operation which can be revealed in many different ways. For example, a number of approaches to binarization are described in [M. Sezgin, B. Sankur, “Survey over image thresholding techniques and quantitative performance evaluation”, Journal of Electronic Imaging 13(1), 146-165 (January 2004)].
It is known a patent application WO2009140738 A1 titled “Image data processing” and aimed at creating a rock model based on different imaging techniques for extracting an information about sample. Main disadvantage of this application is presence of 2D-3D image registration as necessary step for building the 3D digital model accounting pore space and mineral distribution.
Another disadvantage is necessity of making thin sections as sources for 2D mineral maps from SEM or optical microscopy. It is known that making this sections is destructive at microscale. This makes their 2D-3D registration procedure nearly non-applicable in real life. Furthermore, preparing thin sections from the sample after X-Ray microCT destructs original sample, so no further investigations with original sample are possible.
Suggested method provides digital representation of rock's internal structure. Besides, it does not include 2D-3D registration for 3D building the 3D model of the core.